This K&N Stubstack is designed to be used with Holley 390-850 cfm square bore carbs (4150/4160), that have had the choke horn milled off and still use an air cleaner. Eliminates the need for hand polishing and radius work. Will work with all K&N Custom Chrome Racing Air Cleaners, including Tall Velocity Stack Air Cleaners. The K&N Stubstack is designed to increase the airflow of carburetors by decreasing the restriction around the choke horn.